Coastal Vulnerability Assessment Based on Multi- Hazardous Events. Case study: Northwestern Coastline of Guinea-Bissau (NC-GB)

11 Guinea-Bissau coastlines are found highly vulnerable to coastal hazards, and this vulnerability will likely increase 12 under future climate changes scenarios. In addition, the multi-hazardous assessment studies have not yet been 13 conducted to clarify the status of vulnerability index to coastal hazards. Therefore, we integrated eight bio14 geophysical parameters and elaborate a comprehensive Coastal Vulnerability Index to coastal hazards, stablish the 15 rate of sea-level rise and determine the role of coastal habitats in protecting the shorelines in the Northwestern 16 Coastline of Guinea-Bissau, by using the GIS and Coastal Vulnerability Index of InVEST Model. The study found 17 that, out of 87 km of the studied coastlines, nearly 45 km lie in high to very-high vulnerability index. 17 km are 18 found in a moderate vulnerability index and 25 km are found at low to very-low vulnerability index. The main 19 responsible for high vulnerability registered in Zone-B were the wind and wave exposure, as this coastline is highly 20 exposed to sea. The other reason was the storm surge and sea-level that rises 8.79/year, motivated by low coastal 21 elevation. Mangrove ecosystem that are largely found in Zone-A, play very important role in protecting shoreline 22 from coastal hazards with value 0.61, followed by forest and sand dune that are found mostly in Zone-C with 0.49 23 and 0.4 respectively, and saltmarsh that are relatively found in Zone-B with value 0.32. These findings can assist 24 coastal managers in cost-effective adaptation plans, provide a scientific basis for sustainable coastal management 25 and guidance for ecological conservation in coastal regions. 26


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Various coastal regions are vulnerable to coastal hazards due to their proximity to the sea (Sahoo and   Although, no study on multi-hazardous assessment by integrating bio-geophysical 65 parameters in this case "the natural habitat, geomorphology, relief, bathymetry, wind, wave, storm surge and sea-66 level rise" has been conducted to clarify the status of vulnerability index of this coastal areas. Therefore, in this 67 study, we intend to 1) elaborate a comprehensive Coastal Vulnerability Index to coastal hazards, 2) stablish the rate  The study area is geographically located in the NC-GB, between 12° 16′ 14″ N, latitude and 16° 9′ 57″ W 75 longitude ( Fig. 1), covering an area of 1,035.1 km2 with extended coastlines nearly 87 km long (Sintra 2016). The 76 climate is tropical with temperatures variation from 20 °C (68 °F) to 30 °C (86 °F) in April to May, and annual 77 average rainfall of about 2,000 mm. The study area is divided into three important ecological zones, considering

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conceptual framework of vulnerability index on the basis of an analytical hierarchy approach is presented in (Fig. 2).  package was assigned to "5000" in the model to determine the radius in meters around each shoreline point 117 (Naturalcapitalproject 2020). The ArcGIS 10.5 was used to create this raster input. protecting the shoreline from coastal hazards. The Google-Earth was used for on-screen digitization considering 137 nearly 1 to 3 km from the shorelines to inland (Fig. 3d). The digitized data were exported to ArcGIS 10.5 for data 138 processing. To add this variable into the CVI model, we provided a habitat table (CSV) to guide the model on the 139 habitat inputs. The table contained headings "id" (text string without spaces applied to define the habitat uniquely)

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"path" (the file name and habitat location in the layer of GIS), "rank" (value from 1 to 5, as described in Table 4 141 "protection distance" (distance in meters beyond which this habitat will provide).
142 Table 2 is fixed here.

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The CVI model computed a final natural habitat ranking for that point with the following formula: Where means the rank of habitats and N means the number of habitat types.  where: means the average wind speed in meters per second of the ten percent maximum in the ℎ equiangular 164 section. means the percentage of the wind speed at the record that blows in the direction of the ℎ sector. is

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The coastal areas exposed to the open sea commonly suffer greater wave exposure than sheltered ones, due 169 to the fact that winds blow from a considerable distance, generate more giant waves (NOAA 2009

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For ocean waves, the average weighted power was calculated as: Where [ ] means a single function for all sixteen equiangular sectors of the Heaviside pitch wind. Then,

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gets the average of the highest ten percent of the wave power index ( ) that were seen in the angular section 177 orientation, with the average of the time percentage ( ) when such waves were seen in that section. For waves 178 produced locally by the wind, was calculated as: Where [ ] is the opposite of the definition in Equation (7), meaning that only accumulate along rays 181 that do not reach max fetch distance, means the total out over the sixteen wind sectors of the wave power that   199 Table 3 is fixed here.

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In the procedure, millimeter conversion of the data was carried out, the averaged high tide values taken 222   The CVI results and description of the selected eight parameters that impact the coastal structures of the 233 study area are presented below. The study area was divided into three important zones based on their ecological 234 characteristic. Each parameter was assigned to a ranking one to five according to the alternative classification 235 scheme given in Table 5. In this study, the high and very-high Exposure Index was largely occurred in Zone-A and

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B with a CVI model result 3.89% and 3.20, respectively. The very-low EI was mostly noted in Zone-C with value of 237 1.30 and the moderate EI was both identified in Zone-A and C of our study area with value 2.50 (Fig. 4).

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Zone-B recorded the highest vulnerability index due to its highly exposure to wind and wave (Fig. 5). Low 244 relief of the coastlines made zone-A more vulnerable to storm surge and sea-level rise, however, the coastal habitats 245 found in these areas mainly mangroves, play very important role in protecting the coastlines from coastal hazards

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From this analysis, relative wave exposure on the coastlines of the study area is shown. We found that 277 almost two third of the coastlines are highly exposed to wave. Nearly 47 km of the coastlines under ranking 4 to 5 278 lie in high to very-highly vulnerable areas (Fig. 7d). Roughly 24 km under ranking 3 were located in a moderate 279 vulnerable area, and approximately 17 km of the coastlines under ranking 1 to 2 lie in a low and very-low vulnerable 280 areas. The registered high and very-high vulnerability index in Zone-B and C, were due to their exposition to

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Atlantic Ocean and continental shelf. The low and very-low vulnerable areas found in Zone-A has to do with notable 282 sheltered areas by the river Cacheu and estuary (Fig. 8). to very-low storm surge exposure. The high and very-high vulnerability registered in Zone-A were due to low 289 elevation and slope of the coastlines (Fig. 3a-b). The registered low and very-low vulnerable areas were due to 290 relatively high elevation of this coastline.
8 river tide station and found a significant change rate of 8.79 mm/year (Fig. 6). This rate of calculated change in sea-295 level is higher than the global predicted rate of sea-level change of 3.1 mm/year (Bhuiyan and Dutta 2012).

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Therefore, in this analysis, ranking 5 determines the areas with a very-high vulnerability to sea level rise, while rank 297 1 determines the areas with a very-low vulnerability. Approximately 41 km (47.13) under rank 4 to 5 lie in high to 298 very-highly vulnerable areas (Fig. 7f)   Zone-A, covering mostly mangrove that were ranked 1 in habitat

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The qualitative method provides us with useful data to verify the vulnerability index in the NC-GB. Over 315 ten sites were observed, but we only seven specifics were highlighted and we found similarities between CVI model 316 results and field observations results. As shown in Fig. 8, the zone-A presented a high vulnerability index of surge 317 and sea-level rise as confirmed in the image 3 and 7 of observed places. Our CVI results indicated that zone-B is 318 highly vulnerable to wind and wave due to its exposition to open sea as confirmed in image 6. We also observed 319 that, Zone-C is slightly vulnerable to coastal hazards due to its relatively high elevation supported by

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Significant coastal dunes and beach crest plains characterize these sandy shores, as largely found in Zones-C of 360 study area.

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In this study, we observed that poor coastal management combined by the socio-economic situation and  index registered in were due to the high wind and wave exposure, as this coastline is highly exposed to open sea.

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The other reasons were the storm surge and sea-level that rises 8.79/year, due to low coastal relief and flat slope, 387 associated with inconsistent coastal habitats in the shorelines. The Zone-A, shows to be more resilient than Zone-B 388 and C, due to the largely mangrove ecosystem that play very important role in protecting shoreline from coastal 389 hazards. The Zone-B and C, deserves special attention from the conservation bodies and coastal community.

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These findings, will assist policy makers or coastal managers in defining strategies that aimed to minimize  The recommended management strategies to protect the NC-GB based on the current CVI result are 408 summarized in Table 6. For management recommendation, the study area is divided into three zones (Fig. 4)

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according to their specified vulnerability index situation. The detailed for coastal management is highlighted below:

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2. Zone-B: these shorelines are widely exposed to sea and highly vulnerable to wind and wave exposure, due 416 to low spatial elevation associated with lack of consistent habitats and geomorphologies in the shorelines 417 ( Fig. 7a-b). Promoting natural habitat conservation, mangroves restoration and native wood forests   The ow diagram of the coastal vulnerability approach used in this research.

Figure 4
Exposure Index values in the zone-A, B and C along the study area, de ning very high to very low vulnerability index.

Figure 5
Exposure index's variation of physical parameters in the Northwestern Coastline Guinea-Bissau. Wind, sea-level rise, storm surge and wave with higher exposure index in the study area.   Field observation sites in the study area. Image-1: Coastal dune formed by the accelerated accretion; Image-2: Coastal infrastructure degradation in Nhiquim shoreline; Image-3: Sea-level rise in Jobel community; Image-4: coastal elevation in Varela shoreline; Image-5: Flat rocky structure in Varela shoreline; Image-6: Cultivation and mangrove land in Igim community; Image-7: Landscape and sea-level in Bolol community.

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